A passive optical network (PON) includes a passive optical power splitter/combiner that feeds individual branching fibers to end users. The PON also has a tree topology that maximizes coverage with minimum network splits, thus reducing optical power loss. In addition, a common fiber feeder part of a PON is shared by all optical network units (ONUs) with terminating branching fibers. Moreover, traffic sent downstream from an optical line terminal (OLT) at a local exchange is simply broadcast by an optical power splitter to every ONU. Sending traffic from an ONU upstream to a local exchange, however, requires accurate multiple access techniques in order to multiplex collision-free traffic generated by the ONUs onto the common feeder fiber.
At least four major categories of multiple access techniques for fiber have been developed. These techniques include: Time Division Multiple Access (TDMA), SubCarrier Multiple Access (SCMA), Wavelength Division Multiple Access (WDMA), and Optical Code Division Multiple Access (OCDMA).
As the user bandwidth demands are ever increasing, PONs with larger bandwidth are required in the future. Common approaches to improve the bandwidth in PONs include higher data rate Time Division Multiplexing (TDM) PONs and Wavelength Division Multiplexing (WDM) passive optical network (WPON) systems in which multiple wavelengths are used. TDM and WDM approaches may be combined as well. As an example, ITU-T has standardized a higher speed TDM PON (ITU-T G.987, XG-PON, which can provide 10 Gigabits per second (Gb/s) downstream and 2.5 Gb/s upstream.
In a WPON network, each ONU uses a wavelength channel to send packets to an OLT at a local exchange. In addition, the wavelength channel constitutes an independent communication channel and may carry a different signal format from other wavelength channels carried by other ONUs connecting to the OLT.
Conventionally, a WPON network is designed to make each hardware unit at each endpoint, as well as each wavelength selective multiplexing element in the network, tune to a unique wavelength. This design works for wavelength independent power splitting PONs. However, a network with such a design is difficult to manage and prone to errors. One of conventional ways to improve performance of such a design is to implement “colorless” end-point equipment. In a colorless WPON network, an ONU has no intrinsic channel assignment. The ONU obtains a channel assignment by virtue of what fiber the ONU is attached to on the network. This typically assumes that the network uses a WDM device as a splitting element. The physical effects used in this type of network design are either injection locking of a broadband laser source, or reflective modulation of downstream light.
A prominent feature in GPON and XG-PON systems is the bandwidth asymmetry for upstream and downstream transmission. However, future multimedia applications such as video conference and point-to-point (P2P) streaming may require symmetric bandwidth for downstream and upstream. A simple way to improve the upstream bandwidth is to use higher transmission rates. For example, 10 G PON (IEEE 802.3av) uses 10 Gb/s upstream transmission. However, an ONU with a higher transmission rate requires a more expensive transmitter. An alternative approach is to use WDM to improve the upstream bandwidth. That is, ONUs may transmit upstream signals with different wavelengths while the OLT uses multiple receivers to recover the upstream data. For this scheme, the cost increase for ONUs is smaller, but service providers would have to keep track of different ONUs with different wavelengths, and ONUs may need temperature control to keep their wavelengths stable. There is an ongoing need to develop WDM schemes that operate over a power splitting PON infrastructure that does not require wavelength selected ONUs.